The invention relates to a hardenable copper alloy used for manufacturing casting rolls and casting wheels that are subjected to changing temperature stresses.
A world-wide goal, particularly of the steel industry, is to cast a semi-finished product as close as possible to the dimensions of the final product in order to economize on hot and/or cold working steps. Since about 1980, a series of developments have evolved to cast semi-finished products close to final dimensions, for example the single- and double-roll continuous casting methods. When these casting methods are utilized for casting steel alloys, nickel, copper, and their alloys, very high surface temperatures arise in the area of the water-cooled cylinders or rolls where smelt is poured in. For example, these temperatures lie in a range of 350.degree. to 450.degree. C. when steel alloy is cast, and the casting rolls consist of a CuCrZr material having an electric conductivity of 48 m/.OMEGA./mm.sup.2 and a thermal conductivity of about 320 W/mK.
Until now, materials based on CuCrZr have been used primarily for highly thermally stressed continuous casting molds and casting wheels. When these materials are used for casting rolls, the cooling of the casting rolls causes the surface temperature of the region immediately ahead of the pour-in area to drop off cyclically with every revolution, to about 150.degree. to 200.degree. C. On the other hand, on the cooled side of the casting rolls, the temperature remains largely constant during the rotation, at about 30.degree. to 40.degree. C. The temperature gradient between the surface and the cooled side, combined with the cyclical change in the surface temperature of the casting rolls, produce considerable thermal stresses in the surface area of the roll material.
Fatigue tests carried out on previously employed CuCrZr material, having an expansion amplitude of .+-.0.3% and a frequency of 0.5 Hz, which correspond to a 30 r.p.m. speed of rotation for the casting rolls, indicate that at a maximum surface temperature of 400.degree. C., which corresponds to a wall thickness of 25 mm above the water cooling, one can expect a lifetime of 3000 cycles before the formation of cracks occurs. The casting rolls would, therefore, have to be reworked after a relatively short operating time of about 100 minutes to remove surface cracks. Replacing the casting rolls necessitates stopping the casting machine and interrupting the casting operation.
Another disadvantage of the CuCrZr material is its Brinell hardness of about 110 to 130, which is relatively low for this application. Steel splashes cannot be avoided in the single- or double-roll continuous casting method in the region immediately ahead of the pour-in area. The solidified steel particles are then pressed into the relatively soft surface of the casting rolls, thus adversely affecting the surface quality of the 1.5 to 4 mm thick cast bands.
The lower electrical conductivity of a known CuNiBe-alloy with an admixture of up to 1% niobium leads to a higher surface temperature, compared to a CuCrZr alloy, since the electrical conductivity is inversely proportional to the thermal conductivity. The surface temperature of a casting roll made of the CuNiBe-alloy, compared to a casting roll of CuCrZr with a maximum temperature of 400.degree. C. on the surface and 30.degree. C. on the cooled side, will increase to about 540.degree. C.
Generally, ternary CuNiBe-, or rather CuCoBe-alloys do in fact exhibit a Brinell hardness of over 200. However, the electric conductivity of the standard types of semi-finished products manufactured from these materials, such as rods for manufacturing resistance welding electrodes, or sheet metal and bands for manufacturing springs or lead frames, reaches values lying only in the range of 26 to 32 m/.OMEGA./mm.sup.2. Under optimal conditions, a casting roll surface temperature of only about 585.degree. C. would be reached using these standard materials.
Finally, for the CuCoBeZr or CuNiBeZr alloys, generally known from the U.S. Pat. No. 4,179,314, there is no indication that conductivity values greater than 38 m/.OMEGA./mm.sup.2 are achievable in conjunction with a minimum Brinell hardness of 200 when alloy components are selectively chosen.